Make custom REPLs and the `-i` flag work together

[bswck]

Bug report

Bug description:

Combining a custom REPL with -i is currently ramshackle, because pymain_run_python doesn't know that something (either a file, or a module, or stdin) acted as a REPL and therefore a new one right after executing the original code isn't desired:

cpython/Modules/main.c

Lines 681 to 697 in 5f91d5d

	if (config->run_command) {
	*exitcode = pymain_run_command(config->run_command);
	}
	else if (config->run_module) {
	*exitcode = pymain_run_module(config->run_module, 1);
	}
	else if (main_importer_path != NULL) {
	*exitcode = pymain_run_module(L"__main__", 0);
	}
	else if (config->run_filename != NULL) {
	*exitcode = pymain_run_file(config);
	}
	else {
	*exitcode = pymain_run_stdin(config);
	}
	pymain_repl(config, exitcode);

This leads to broken expectations around python -i -m pdb or python -i -m asyncio:

Broken output of python -i -m asyncio

❯ ./python -i -m asyncio
asyncio REPL 3.15.0a0 (heads/main:5f91d5d9a41, Oct 12 2025, 22:21:59) [GCC 15.2.1 20250808 (Red Hat 15.2.1-1)] on linux
Use "await" directly instead of "asyncio.run()".
Type "help", "copyright", "credits" or "license" for more information.
>>> import asyncio
>>> exit
exiting asyncio REPL...
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/home/bswck/Python/cpython/Lib/asyncio/__main__.py", line 240, in <module>
    sys.exit(return_code)
    ~~~~~~~~^^^^^^^^^^^^^
SystemExit: 0
>>> exit

Broken output of python -i -m pdb

❯ ./python -i -m pdb
usage: python -m pdb [-h] [-c command] (-m module | -p pid | pyfile) [args ...]

Debug the Python program given by pyfile. Alternatively,
an executable module or package to debug can be specified using
the -m switch. You can also attach to a running Python process
using the -p option with its PID.

Initial commands are read from .pdbrc files in your home directory
and in the current directory, if they exist.  Commands supplied with
-c are executed after commands from .pdbrc files.

To let the script run until an exception occurs, use "-c continue".
To let the script run up to a given line X in the debugged file, use
"-c 'until X'".

options:
  -h, --help            show this help message and exit
  -c, --command command
                        pdb commands to execute as if given in a .pdbrc file
  -m module
  -p, --pid PID         attach to the specified PID
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/home/bswck/Python/cpython/Lib/pdb.py", line 3650, in <module>
    pdb.main()
    ~~~~~~~~^^
  File "/home/bswck/Python/cpython/Lib/pdb.py", line 3568, in main
    sys.exit(2)
    ~~~~~~~~^^^
SystemExit: 2
>>> exit

Broken output of python -i -m code

❯ ./python -i -m code
Python 3.15.0a0 (heads/main:5f91d5d9a41, Oct 12 2025, 22:21:59) [GCC 15.2.1 20250808 (Red Hat 15.2.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> exit()
now exiting InteractiveConsole...
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/home/bswck/Python/cpython/Lib/code.py", line 397, in <module>
    interact(banner)
    ~~~~~~~~^^^^^^^^
  File "/home/bswck/Python/cpython/Lib/code.py", line 383, in interact
    console.interact(banner, exitmsg)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/home/bswck/Python/cpython/Lib/code.py", line 287, in interact
    raise e
  File "/home/bswck/Python/cpython/Lib/code.py", line 277, in interact
    more = self.push(line)
  File "/home/bswck/Python/cpython/Lib/code.py", line 325, in push
    more = self.runsource(source, filename, symbol=_symbol)
  File "/home/bswck/Python/cpython/Lib/code.py", line 75, in runsource
    self.runcode(code)
    ~~~~~~~~~~~~^^^^^^
  File "/home/bswck/Python/cpython/Lib/code.py", line 91, in runcode
    exec(code, self.locals)
    ~~~~^^^^^^^^^^^^^^^^^^^
  File "<console>", line 1, in <module>
  File "<frozen _sitebuiltins>", line 26, in __call__
SystemExit: None
warning: can't use pyrepl: I/O operation on closed file
>>> exit()

This is an easy fix. Well, we could avoid the problem by just documenting here that combining this flag isn't supported and perhaps guard against sys.flags.interactive in custom REPLs, but I dont think it's reasonable at all in this case.

We can simply introduce a flag (like, sys._running_custom_repl) or a function (like, sys._enter_custom_repl, that could raise an error if re-entered) that would inform the pymain_run_python function back that something already acted as the REPL for this runtime. The scope of that feature would be tightly coupled to pymain_run_python only! The feature could be then used in interact() or wherever it fits the most.

I don't have a clear proposal regarding the constraints that sys._enter_custom_repl would check; that's a work in progress and I'm open to suggestions.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

[bswck]

OK, after thinking through this, this one is invalid either.

I wouldn't say it's a bug, and this works as intended. And I don't think -i combined with custom REPL code could be expected to be useful, although it might be confusing if someone doesn't read the documentation first. We can add an additional note about this in the docs or raise a warning upon sys.flags.interactive in code interactive interpreter, but otherwise I don't think this is something that's "to be fixed" in the first place.

Sorry for the hassle! I'll think harder next time before opening an issue :-)